A memory can store information in an array of transistors. A threshold voltage associated with transistors is the gate voltage required to form a conducting channel. For example, an n-channel enhancement-type metal oxide semiconductor field-effect transistor (NMOS transistor) includes a source region, a drain region, and a gate region. When a positive voltage is applied to the gate of the NMOS transistor, the positive voltage attracts electrons from the source and drain regions into a channel region of the NMOS transistor. When a sufficient number of electrons accumulate near the surface of the substrate, a channel for current flow from the drain region to the source region is created. The threshold voltage is the voltage at which a sufficient number of mobile electrons accumulate into a channel region to form a conducting channel.
Each memory cell is connected to a word line and a bit line. Accordingly, each memory cell can be addressed by a word line signal and a bit line signal. Each memory cell can be selected for reading, verifying, programming, or erasing by activating the word line and the bit line. For example, programming is obtained by applying a voltage to the selected word line, which connects to the gates of the transistors, and biasing the bit line, which connects to the drain regions of the transistors. A read or verify operation is generally performed by applying a selected word line voltage to the memory cell and sensing the current flowing through the memory cell by way of its respective bit line.
Since the current flowing through the memory cells varies with temperature, the threshold voltage varies with temperature. Similarly, the selected word line voltage also varies with temperature. However, the selected word line voltage and the threshold voltage vary differently with temperature change. In other words, when compared to the threshold voltage, the selected word line voltage varies with temperature at a different rate or has a different temperature coefficient (volts/temperature). Since the reliability of read and verify operations strongly depends on selected word line voltage and threshold voltage distributions, such rate differences result in unreliable or erroneous read and verify operations. Non-volatile memories that store more than one bit in a transistor associated with a memory cell are particularly sensitive to threshold voltage distributions because the non-volatile memories are required to have a fine control of threshold voltage distributions of programmed memory cells.
As a result, there is a need for continuing efforts to improve the reliability of read and verify operations.